There is known an ink jet recording apparatus that ejects droplets of ink from a nozzle of an inkjet head and deposits them onto a recording paper, so as to record an image represented by ink dots. There is also known a type of ink jet recording apparatus which performs duplex recording by reversing the front and rear surfaces of a recording paper. Typically, a conventional ink jet recording apparatus with duplex recording capability feeds a recording paper in a forward direction, and ejects ink droplets to the front side (first surface) of the recording paper during the feed, so as to record a first image. When the deposited ink has dried on the first surface, the recording paper is fed in a backward direction, and reversed back to front. Then, the recording paper is fed again in the forward direction, and ink droplets are ejected to the back side (second surface) of the recording paper during the feed, so as to record a second image.
A certain drying time is necessary after the first surface recording process because, if the recording paper was reversed before the deposited ink had dried and fixed completely on the first surface, the undried ink droplets would be pulled by a roller or a guide that touched the first surface, and blur the first image. This drying time also prevents the ink from transferring to the roller or the guide to smear the next recording paper.
The conventional ink jet recording apparatus uses a long drying time so that the recording paper is reversed when the deposited ink on the first surface has completely dried and fixed. However, the drying time is not changed in most cases even for an image with a few ink droplets (an image with low average density), and thus it takes a long time to complete duplex recording. An ink jet recording apparatus to solve this drawback is disclosed in U.S. Pat. No. 6,149,327. This ink jet recording apparatus measures total volume of ink droplets necessary for the first surface of a recording paper, and determines the drying time based on the measured volume.
Ink droplets dry and fix slower when deposited densely to overlap with each other than when deposited separately. The above ink jet recording apparatus is, however, unable to judge whether the ink is deposited densely or separately, and would reverse the recording paper before the ink has not dried completely on the first surface. Japanese Patent Laid-open Publication No. 2005-125750 discloses an ink jet recording apparatus to solve this drawback. This ink jet recording apparatus divides an image to be recorded on the first surface into several regions, and measures volume of ink droplets necessary for each region, and determines the drying time based on the measured volume.
Ink droplets on a recording paper start drying and fixing as soon as they are deposited. In other words, the time it takes to record an image way down to the rear end of a recording paper can be used as the drying time for the deposited ink droplets on the front end of the recording paper. Although the ink jet recording apparatus of the publication No. 2005-125750 regards the drying time for the most-ink-droplets region as the drying time for the first surface, it cannot reduce the drying time when the most-ink-droplets region lies on the rear end of a recording paper. In other words, the recording time cannot be used in full measure as the drying time.